Parallel metering systems have been used in many industrial turbine applications. These systems provide accurate fuel placement to multiple combustor locations via multiple metered flow paths arranged in parallel. In such systems, flow is initially provided by one or more pumps. This initial flow is then divided across the multiple metered flow paths arranged in parallel to distribute the same to multiple combustor locations.
As commercial aircraft combustion systems become more complex in order to provide improved fuel efficiency and reduced emissions, the flexibility of a parallel metering system may offer significant benefits. Positive displacement pumps are often preferred for turbine engines due to their good efficiency and high reliability. Parallel metering systems used in industrial turbine engine applications typically include a pressure regulated positive displacement pump that supplies flow to the multiple metered flow paths arranged in parallel. The pump pressure regulation system typically maintains the pump discharge pressure at a constant pressure that is high enough to meet the pressure needs of the metered flow paths for all operating conditions.
Unfortunately, this discharge pressure may be significantly higher than the requirements of the metered flow path. Operating the pump at pressures higher than required leads to undesirable additional pump heat input to the fuel system. Thermal efficient fuel pumping is typically required to meet the aircraft engine operational requirements. Maintaining the positive displacement pump at high pressure for all operating conditions is likely not acceptable for most aircraft engine applications.
Some proposed parallel metering systems for aircraft engines consider controlling the pump discharge pressure to attempt to more closely match the system pressure needs by estimating the required pressure and then using an electrically driven pressure regulator to set the desired pressure. These systems require that the estimated pressure include all system variation and life impacts; this stack up can lead to higher than desired system pressure.
Another pit fall of other proposed parallel metering systems is that they do not allow for a single flow meter downstream of the high pressure pumping system and upstream from the parallel metering paths that measures total engine burn flow.
Accordingly, there is a need in the art for a parallel metering system that does not require substantially over-sizing the pump and its attendant discharge pressure in an effort to accommodate various systemic pressure demands, as well as a system that allows for the utilization of a flow meter to measure total engine burn flow.
The invention provides such a parallel metering system. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.